This invention relates to particle detection and, in particular, to particle detection achieved by a condensation nucleus counter that implements a multi-directional fluid flow system to enlarge condensation nuclei of submicron particles entrained in a fluid stream such that they can be detected and counted.
The current technological trend toward the design of electronic and other devices with increasingly smaller device dimensions has created an increased need for detecting a presence of submicron diameter contaminant particles in facilities where such devices are manufactured. Such need is especially urgent in the field of microelectronic device fabrication, in which the presence of particulate contaminants results in a significant reduction in product yield.
Use of a condensation nucleus counter (CNC) represents one method of detecting the presence of submicron particles. In CNC devices, a process of diffusion thermal cooling of a fluid stream carrying submicron particles enlarges the diameters of the particles to sizes that allow their detection using conventional techniques. During thermal diffusion cooling, a submicron particle-carrying fluid sample is passed over a heated pool of volatile liquid, resulting in saturation of the fluid sample with volatile liquid vapor. The resulting vapor-fluid sample mixture is then cooled by thermal diffusion from the cold walls of a condenser. This cooling results in condensation of the vapor onto the surfaces of the submicron particles, thereby enlarging them to form droplets of sufficient size to allow optical detection of the particles.
The resolution afforded by a CNC device depends upon droplet size. Droplet size is a function of condensation time, which is dependent on the flow rate of the vapor-fluid sample mixture and the length of the flow path through the condenser. Specifically, reducing the flow rate causes maximum supersaturation to occur at a location closer to an inlet tube of the CNC device. A shorter flow path facilitates the design of a lightweight, lower cost instrument. However, reduction of the flow rate also results in increased processing time, which leads to inefficient and more costly particle detection.
Thus, a first problem encountered in prior art CNC devices is an inability to reconcile these competing benefits and detriments to arrive at a lightweight, low-cost instrument that performs efficient and cost-effective particle detection. A second problem with prior art CNC devices is that the submicron particles have a tendency to bounce off the impaction stage and become re-entrained in the flow system of CNC devices. The number of incidences of so-called xe2x80x9cparticle bouncexe2x80x9d events increases with flow rate. One prior art method of minimizing particle bounce is described in U.S. Pat. No. 5,659,388 and entails applying grease to the impaction stage. This method is of limited utility because it introduces contaminants into the CNC device and thereby renders it unsuitable for use in many high-technology industries including microelectronic device fabrication.
What is needed, therefore, is a lightweight, low-cost CNC device implementing an improved method for efficiently and cost-effectively detecting the presence of submicron particles while limiting the incidence of particle bounce.
An object of the invention to provide a compact, lightweight CNC device that uses a multi-directional, and preferably radial, fluid flow system to enlarge condensation nuclei of submicron particles carried by a sample fluid stream for detection.
Another object of the invention is to provide such a device that forms a radially expanding fluid flow path in a working fluid saturation region to increase diffusion time of the working fluid into the sample fluid to form a saturated fluid mixture.
A further object of the invention is to provide such a device that implements a radially convergent fluid flow in a condensation region to decrease the length of the fluid flow path.
Still another object of the invention is to provide such a device that incorporates a conical geometry in the condensation region to form an annular fluid flow path of decreasing cross sectional area to increase the velocity of the sample fluid stream as it flows through the condensation region.
Yet another object of the invention is to provide such a device that operates within a range of above ambient fluid pressures to count with high resolution a greater percentage of entrained particles.
The present invention is a condensation nucleus counter (CNC) device implemented with a multi-directional, and preferably radial, fluid flow system. A preferred embodiment of the CNC device includes a saturation region and a condensation region configured in a compact, conical geometry. The saturation region includes a working fluid pool and an annular fluid pool separated by a working fluidxe2x80x94working vapor interface. The saturation region is heated to form from the pool of working fluid a vapor that enters the annular fluid pool. A preferred working fluid is a liquid, such as water, which evaporates to form water vapor. The saturation region includes a tubular inlet that exhausts a sample stream of air containing small-diameter particles into the annular fluid pool above a working fluid surface, which defines the working fluidxe2x80x94working vapor interface. The stream of air flows out radially from the inlet, expands across the working fluid surface, and mixes with the working vapor to become saturated in the saturation region. The flow velocity of the air stream slows as it expands and thereby increases the diffusion time of the working vapor to form the saturated fluid mixture.
The saturated fluid mixture then flows through an annular inlet to the condensation region and out of the CNC device through a tubular outlet. The condensation region is defined by spaced-apart inner and outer radially converging walls having a conical geometry, thereby forming an annular flow volume of decreasing cross sectional area in the direction of fluid flow from the inlet to the outlet. The working vapor condenses on the small-diameter particles to enlarge their optical scattering cross sections with less tendency to condense on the inner and outer walls of the condensation region. Thus, particles having diameters on the order of nanometers are the seeds for droplets having diameters on the order of microns.
The measurement resolution of the CNC device depends of the sizes of the particles to be detected. Particle size is a function of the fluid flow rate and the length of the flow path through the CNC device. The radial fluid flow system and conical geometry of the condensation region ensure that the residence time of the particles within the CNC device is great enough to achieve supersaturation of the particles in a compact device. An alternative embodiment receiving and processing a sample stream at above ambient pressures counts a greater percentage of particles because the higher fluid pressure contributes to a minimization of working vapor dropout onto the interior flow surfaces of the condensation region.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.